Long drain oils have been desired than before from the viewpoints of recent demands such as effective use of resources, decrease of waste oils, and reduction of costs incurred by the users.
The inventors of the present invention found that in order to enhance the long drain properties of a lubricating oil, a low sulfur-content lubricating oil composition containing a metal salt of a monothiophosphate ester-based compound reduced in sulfur in the molecules or that of a phosphate ester-based compound or phosphonate ester each containing no sulfur in the molecules instead of a sulfur-containing anti-wear and -oxidation agent such as zinc dialkyldithiophosphate, had excellent base number retaining properties and oxidation stability leading to long drain properties while maintaining anti-wear properties and was capable of exhibiting high temperature detergency and low friction properties. The inventors had already filed patent applications of these inventions which were assigned to Japanese Patent Application Nos. 2002-015351 and 2002-246975, respectively.
While zinc dialkyldithiophosphates are liquid at ordinary temperature, these metal salts of phosphorus compounds are solid at ordinary temperature and thus have problems that they are not only poor in handling properties but also low in dissolubility in the lubricating oil or require a long period of time to be dissolved as they are. Therefore, the problems make it difficult to mass-produce a lubricating oil composition containing these metal salts of the phosphorus compounds industrially efficiently.
In view of the foregoing situations, the present invention is intended to provide a technique for liquefying metal salts of phosphorus compounds with low dissolubility in a lubricating oil so as to be dissolved therein efficiently for a short period of time, thereby producing industrially efficiently a lubricating oil composition containing a metal salt of a specific phosphorus compound, with excellent properties such as long drain properties.